The following condensed list of expressions are all currently
mislabelled as valid by `canonicalize_license_expression()`.

    ( ) or MIT
    MIT and ( )
    ( ) and MIT or MIT
    ( ) with ( MIT )
    ( ) with ( ) or MIT
    MIT with ( ) with ( ) or MIT

The trick of converting license expressions to Python statements runs
afoul of `()` being an empty tuple (i.e. valid syntax and falsy).
Depending on how `()` is combined with real `False`s, it may or may not
get caught by the subsequent `eval(python_expression) is False` check:

    MIT or ()  ->  False or ()  ->  ()     # Caught
    () or MIT  ->  () or False  ->  False  # Not caught

Consider a license expression invalid if a `)` comes immediately after a
`(` token. This also removes the one case where the `eval()` would
return something other than `False` so that `eval()` can now be
downgraded to a `compile()` (which I find very anxiety relieving even
though I can't think of any way the old `eval()` could ever have been
exposed to more than `False`s or `()`s).